In drilling of wells, drilling fluid is generally circulated through a drill string and drill bit and then back to the surface of the wellbore being drilled. At the surface, the fluid may be processed to remove solids and to maintain desired properties before it is recirculated back to the well. During drilling operations, some amount of this drilling fluid may be lost due to various factors. This loss of drilling fluid may be referred to as lost circulation. Lost circulation is one of the largest contributors to non-productive time in drilling operations. This is particularly true for wells being drilled in complex geological settings such as deep water or highly depleted zones/intervals. Thus, it is important to determine the causes of lost circulation and try to mitigate those factors.
One major factor contributing to lost circulation is the formation of fractures in the wellbore wall. The fractures provide an outlet for the drilling fluid to escape from and thus result in loss of fluids. Loss of circulation due to creation of fractures in the wellbore wall is a major problem in drilling operations, as it is costly and may result in loss of well control. Additionally, if left untreated, undesired fractures could threaten the integrity of the entire wellbore. To prevent or mitigate wellbore losses, an engineering practice referred to as wellbore strengthening may be conducted.
Wellbore strengthening can done using a variety of different techniques. One common wellbore strengthening technique involves sealing existing natural fractures or induced fractures with a lost circulation material, after they have been created. Sealing of fractures in wellbore strengthening generally occurs with materials having properties that are conducive to sealing of the wellbore wall. In general, to conduct a successful wellbore strengthening operation, the width of a fracture at the wellbore wall has to be determined. This allows accurately engineering a lost circulation material having a suitable particle size distribution that can seal the fracture at the wellbore wall.
While sealing of fractures after their formation may be appropriate in some cases, this technique may be less than ideal in other situations. For example, in some instances it may be more efficient to strengthen the wellbore wall such that undesired fractures do not form during drilling. Strengthening the wall may involve increasing the pressure at which an undesired fracture will form in the wellbore wall. The pressure at which a fracture will form generally corresponds to a property referred to as the fracture gradient.
One wellbore strengthening technique involves increasing the fracture gradient of the wellbore wall by intentionally inducing fractures that are then sealed. This has been shown to mitigate future fractures and hinder further fracture propagation. To create induced fractures, mud weight has been used to exert extra pressure on the formation. When pressure exerted by mud weight exceeds the fracture gradient of the wellbore at a particular point in the well, a fracture is created at that point. To control the size of the induced fracture and the increase in the fracture gradient, it may be important to know the precise amount of mud weight to use at a particular location.
To determine what strengthening technique to use for a given wellbore, areas of the wellbore that may be susceptible to fracture formation may first need to be identified and the mud weight at which a fracture may be formed in those areas may need to be determined. Still because of uncertainties associated with drilling operations, it may not always be easy to determine which wellbore strengthening technique to use for a given wellbore or what mud weight or lost circulation material would be the most effective. The following disclosure addresses these and other issues.